Resistance element



H. F. FRUTH RESISTANCE ELEMENT Filed Marsh 14, 193e Nov. l, 1938.

Patented Nov. l, 1938 UNI-TED `STATES PATENT oFFlca Mallory s Co., Inc., indianapolis, Ind., a

corporation of Delaware Application March 14, 1936, Serial No. 68,870

3 Claims.

This invention relates to resistance elements and more particularly to elements having a resistance surface applied thereto for use in variable reslstance devices such as rheostats, radio 5 volume controls and the like. i

An object of the invention `is to produce an improved resistance element for a variable resistance device.

Other objects ofthe invention will be apparent from the description and accompanying drawing taken in connection with the appended claims. i

The invention may be applied to resistance elements of the type comprising a disc, sheet or ring of insulating material or of a conductive material having an insulating surface, the element in each case having a layer of resistance material coated on a surface thereof. The resistance surface may comprise a plurality of layers of the same or of different composition applied to the base. t i

The invention accordingly comprises the features of construction, combination of elements, arrangement of parts, and methods of manufacture referred to above or rwhich will be brought out and exemplified in the disclosure hereinafter set forth, including the illustrations in the drawing, the scope of the invention being indicated in the appended claims. l For a fuller understanding of the nature and objects`of the invention as well as for speclnc fulfillment thereof, reference should be had to the following detailed description taken in connection with the accompanying drawing, in which:

Figure 1 is a face view of a variable resistance device with its cover removed;

Figure 2 is a section on the line 2-2 of Figure 1 with the cover added;

Figures 3 to '1 inclusive illustrate different stages in the manufacture of a resistance element for the variable resistance device;

Figure 8 is a section of a finished resistance (Cl. 20L-73) method ofV procedureand the construction of parts without departing from the spirit of the invention. In the following description and in the claims parts will be identified by specic names for convenience, but they are intended to 5 be as generic in their application to similar parts as the art will permit.`

Referring to the drawing and particularly to Figures 1 and 2 the variable resistance device 20 comprises a base formed of two superimposed l0 discs 2l and 22 of insulation,such as fibre impregnated with a synthetic resin. Discs 2| and 22 each have suitable cuts-out portions, to ac-V commodate other parts of the resistor structure.

A metal cover or cap I9 is secured to the discs 2| l5 26 are provided at the two ends of the resistance surface.

Terminal bolts 21 andv 23 pass` through ring 2.3 and base disc 22 and are ln electrical contact with terminal coatings 25` and 26, respectively. '25

Terminal lugs 23 and 30 are secured by bolts 21 and 28, respectively, underneath disc 22 and project out beyond the baseto provide for external circuit connections to the two` ends of the resistance surface. serve as terminals but likewise clamp 'the ring 23 to base disc 22. L

Discs 2| and 22 are clamped togethei` at their centers by `a metal bushing 32 which passes through these members anda metal disc 33 35 clamped'against the back of disc 2|`by bushing 32. Y

A metal contact ring 3l having an integral terminal lug 35 is mounted against the face of disc 22 within the resistance unit ring 23 and concen- 40 tric therewith. Ring'34`isheld in place by a rivet or eyelet 36 which clamps disc 2| and lug 35 together. A

The variable contact'or structure comprises a shaft 31 rotatable in bushing 32, a contact arm 45 33 secured to the end of shaftA 31 on the face side of said resistance device, a sliding contactor 40 mounted on the end of said arm and adapted to slide on resistance surface 2l onring 23, and sliding contact fingers 39 integral with 50 said arm and adapted to slide on contact ring 34. Shaft 31 is held against axial motion by clip washer 3| fitted `in an annular groove in shaft 31 adjacent the rear end of bushing 32. Contact arm 33 is secured to the end of shaft 31 and inu Bolts 21 and 23 notonly 30 lation 4I the end of said shaft being upset to hold said piece 4| in place. Contact armr38 and piece 4I are clamped together by a rivet pin 42. Pin 42 has a projecting portion 43 for operating a. switch element which may be secured to the cover, -if desired, so that the switch actuating lever is engaged by projection 43. Arm 38 and contact fingers 39 areof spring construction and are held in tension by clip washer 3l so that the sliding contact portions are held in contact with their respective ring surfaces.

In operation, the terminal lugs 29, and 35 are connected into an external circuit in any desired manner. By turning shaft 31 with a knob or other control means sliding contactor 40 will slide on the resistance surface 24 in a circular path. Thus the length of theresistance path, and consequently the magnitude of the resistance in series with terminal lugs 29 and 35 or 30 and 35 may be varied. Conductive coating 26 extends out a sufficient distance from terminal bolt 21 so that contactor may ride up onto` it as it approaches `the terminaL' thereby reducing the resistance to a negligible value. This enables the turning of arm 38 for a few degrees at the end of the stroke to allow for a switch to be opy erated by projection 43 on pin 42, if desired, with control shaftshall vary as a given function of the degree of rotation. The type of resistance vs. rotation curve will, of course, depend on the particular circuit in which the volume control is used. The most generally useful curve is'probably a logarithmic form of curve.

y Resistance elements have been made by coating an absorbent paper disc with a layer of resistance material and then applying a second layer in superimposed fashion over the rst one for onlypart of the length of the resistance path. The units have then been rolled to create a smooth contact surface which wouldproduce less noise in operation.

Howeveryrolling does not distribute the resistance material and" consequently such units have had relatively abrupt jump-off character--` istics at the ends of resistance layers with resultant noise in the electrical circuits. This had been particularly true with units having abase of hard material such as Bakelite fibre. With these units likewise the resistance vs. degree of rotation curves have only roughly approximated the preferred logarithmic curve.

According to one aspect of the present invention a resistance element may be produced which will provide a uniform resistance vs. degree of rotation curve of logarithmic or any other desired form and may be applied to a non-absorbent base as well as to bases of other types.

In accordance with a preferred method of making the resistance element, ring 23 of Bakelized bre (see Fig. y3) is sprayed with resistance `sulated therefrom by a small piece of sheet insu- If a second taper is required the mask is removed and another mask having the required taper openings is used. The second tapered layer 46 (Fig. 5) may preferably cover the rst layer and extend beyond it for the required distance. Additional tapers may be applied when required. Finally an overall coating 41 of resistance material is applied in the same manner resulting in the partially finished unit shown in Fig. 6.

After the finaly resistance coating is applied low resistance terminal coatings 25 and 26, of a highly conductive material such as silver paint, are sprayed through a suitable mask onto the element. In applying this coating, however, the mask is spaced slightly above the surface to be coated. This allows the spray to spread slightly after passing through the mask thereby producing a feather edge `to the coating. 'I'his feather edge is desirable' in that it prevents mechanical roughness and sudden jumps in the electrical resistance when the sliding contactor is moved in Figure 8 in which the thickness of the various` layers is greatly exaggerated.

A feature ofthe above element'is that the softer lower resistance layers are put onto the base first and the harder high-resistance layers are applied over them and serve to protect the softer layers from wear.

The paint for the low-resistance layer 45 may be composed of nely pulverized graphite suspended in denatured alcohol or synthetic alcohol or the lcomposition may be varied to include a certain percentage of lampblack.

For the layer 46, which may preferably be of intermediate resistance, no silver powder is used and a higher proportion of shellac is used. Like,- wise the ratio of lampblock to graphite may be higher. The paints for other intermediate layers, when provided, may have the ingredients variously proportioned to give the desired specific resistance. In general silver produces the lowest resistance, with graphite next and lampblack highest, the addition of shellac tendingin all cases to increase the resistance.

For the topresistance layer 41 lampblack is used with a smaller proportion of graphite'and a higher proportion of shellac. the layers suflicient solvent, such as denatured alcohol, is used to obtain the desired viscosity for spraying. After drying, the top layer, with less graphite and more shellac, will be quite hard and wear-resistant and willprotect the softer layers underneath. Similarly each layer, being harder than the ones beneath it, helps to protect the` Figure 9 shows a modiedform ofsilver paint` coating which may be applied either on top of or underneath the resistance layers. This coating includes a series of strips 48 of silver paint sprayed through a suitable mask, the resistance For applying all material bridging the gaps between adjacent strips. Near the terminal portion 26 the strips 48 are radialbut at the outer end they are skewed a certain amount so that the contactor will bridge two strips. This provides a low resistance taper.

. Figure 10 shows another modification in which the strips of silver paint 48aare skewed near the terminal but are substantially radial at the outer end.

Figurell shows a modification in which a short` tapered low-resistance layer l! is provided adjacent terminal 25 in addition to the tapers at the opposite end. Taper 49 serves to smooth out the change in resistance where the contacter leaves the carbon resistance surface and rides onto the silver terminal 25.

After the resistance and terminal paint coatings have been applied to the base discs they are dried and baked.

For applications where no switch is to be actuated'by the control arm the silver paint terminal 26 can be shorter, that is, similar to terminal 25, since in this instance it is not necessary that the contactor move any great distance Without change in the resistance in the circuit. It is also obviously possible to vary the shape of. tapers used and the relative lengths of the various resistance layers to obtain a resistance vs. degree of rotation curve of any desired shape. It will be noted that the thickness of any layer is constant throughout its area but that it has a variable width due to the taper, which preferably cornes to a point along the middle of the resistance surface.

'I'he life characteristics of the finished units are extremely good. With volume control devices having resstance elements made according to the present invention it has been found that 60,000 rotations of a contact arm forth and back across the resistance surface (120,000 passes) changed the resistance of the units by less than 5%. On humidity tests at 115 F. in air saturated with water vapor the resistance of a unit was changed less than 5% in 24 hours and the resistance was found to return to its original value when the unit was allowed to stand in a normal atmosphere.

While the present invention, as to its objects and advantages has been described herein as carried out in specific embodiments thereof, it is not desired. to be limited thereby but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. A resistance element for a variable resistance device comprising a base of insulating material having an elongated surface, a tapered layer of resistance material on said surface, the base of said taper being adjacent one end of said surface and the point of said taper being intermediate the ends of said surface, and a second tapered layer of resistance material superimposed on said first layer with its base over the base of said ilrst layer and its point beyond the point of said iirst taper. y

2. A resistance element for a variable resistance device comprising a base of insulatory material having a relatively smooth surface, a pair of spaced conductive terminals on said surface, a layer of electric resistance paint on said siu'face in a path extending between said two terminals, said layer comprising a irst area adjacent and in contact with one of said terminals which is of paint of a certain specificV resistance, said area tapering to a point intermediate said terminals and the rest of said resistance path being of paint of a higher specific resistance and contacting the other of said terminals.

3. A resistance element for a variable resistance device comprising a base of insulatory material having a relatively smooth surface, a pair of spaced conductive terminals on said surface, a layer of electric resistance material on said surface in a path extending between said two terminals, said layer comprising a first area adjacent and in contact with one of said terminals which is of a certain specific resistance, said area tapering to a point intermediate said terminals, a second area adjacent and in contact with the other of said terminals which is of a certain specic resistance, said second area tapering to a point intermediate its contacting terminal and the point of said rst area, and the rest of said resistance path being of higher specific resistance than either said rst or said second areas.

HAL F. FRUTH. 

